blue or black background?

Either is generally good...with the black substrate though I would say black background would likely be best. Will be very dramatic and your fish will really pop out. Try getting some coloured paper, put it behind the tank and see what your prefer. edit : with the black you probably want plenty greenery in there or it may look a bit too sombre, unless it is a rift lake cichlid tank in which case the colours of the fish are all you need

In fact I make my backgrounds out of mounting boards cut to fit and propped behind the tank - very easy to change them if you want.

I have had both , I like the black my self , Lots of lighting
I think the black least messes with the fish colors

I think a blue would look good with black substrate! :]

black

I think either would look good actually... the blue would really stand out against the black gravel but all black would really make your fish stand out and not to mention make plants look amazingly green haha! Whatever you prefer really.

I think black looks best with a black substrate.

I would go with the black background. IMO, it would look a lot better with the black gravel than the blue background.